1. Field
This disclosure relates generally to the field of global positioning system (GPS) receivers, and more particularly to a method and apparatus for mitigating satellite signal interference in a GPS receiver.
2. Description of Related Art
As is well known in the field of global positioning system (GPS) design, GPS receivers determine their position by computing the relative times of arrival (TOA) of signals that are simultaneously transmitted from a plurality of GPS satellites (also referred to herein as Space Vehicles (“SVs”)) orbiting the earth. As described in a related U.S. patent, U.S. Pat. No. 6,236,354, issued on May 22, 2001 to Krasner (referred to hereafter as the '354 patent), and incorporated by reference herein in its entirety, GPS satellites transmit satellite positioning data and clock timing data (this data is referred to in the GPS art as “ephemeris” data).
As described in the incorporated '354 patent, GPS receivers determine pseudoranges to various the GPS space vehicles (SVs), and compute the position of the receiver using the calculated pseudoranges and satellite timing/ephemeris data. The pseudoranges are time delay values measured between the received signal from each SV and a local clock signal. The satellite ephemeris and timing data is extracted from the GPS signal once it is acquired and tracked. Acquiring GPS signals can take up to several minutes and must be accomplished using a sufficiently strong received signal in order to achieve low error rates.
The GPS SVs transmit two carrier frequencies called L1, the primary frequency, and L2, the secondary frequency. The carrier frequencies transmitted by each SV are modulated by spread spectrum codes with a pseudorandom noise (PRN) code (also referred to herein as a PN sequence) that is unique to each SV and by the navigation data message. All of the SVs transmit using the same two carrier frequencies, however, their signals normally do not significantly interfere with each other because of the unique PRN code modulation. Because each GPS SV is assigned a unique PRN code and because all of the PRN code sequences are nearly uncorrelated with respect to each other, the SV signals can be separated and decoded using the well known code division multiple access (CDMA) data transmission technique.
The PRN codes available for civilian GPS applications are referred to as C/A (coarse/acquisition) codes, and have a binary phase-reversal rate, or “chipping” rate, of 1.023 MHz and a repetition period of 1023 “chips” for a code period of 1 millisecond. The terms “chip” and “chipping” are used, instead of the term “bit”, to indicate that no data information is contained in the PRN codes. The code sequences belong to a family known as “Gold” codes, and each GPS satellite broadcasts a signal having a unique Gold code.
In simple terms, for a signal received from a given GPS satellite, a receiver multiplies the received signal by a stored replica of the appropriate Gold code contained within its local memory, and then integrates the product in order to obtain an indication of the presence of the signal. This process is termed a “correlation” operation. By sequentially adjusting the relative timing of this stored replica relative to the received signal, and observing the correlation output, the receiver can determine the time delay between the received signal and a local clock. The initial determination of the presence of such an output is termed “acquisition” of the signal. Once signal acquisition occurs, the process enters a “tracking” phase in which the timing of the local reference is adjusted in small amounts in order to maintain a high correlation output.
In order to acquire and track one SV that is in common view with several other SVs, a GPS receiver typically replicates the PRN code for the desired SV together with the replica carrier signal, including Doppler effects caused by Doppler shifts in the carrier frequency. The GPS signal acquisition and tracking process is therefore a “two-dimensional” (code and carrier) signal replication process, wherein both the SV PRN code and carrier frequency are replicated. For example, when acquiring and tracking the SV signal in the “code-phase” dimension, the GPS receiver first replicates the PRN code transmitted by the SV to be acquired, and then the GPS receiver shifts the phase of the replicated code until it correlates with the SV PRN code. Maximum correlation occurs when the phase of the GPS receiver replica code matches the phase of the incoming SV PRN code. Minimum correlation occurs when the phase of the replica code is offset by more than one chip on either side of the incoming SV PRN code.
GPS receivers also detect the SV transmitted signals in a “carrier-phase” dimension. GPS receivers accomplish carrier-phase dimension acquisition and tracking by replicating the SV carrier frequency including Doppler shifts to the carrier frequency. As is well known, the Doppler-induced effects are due to line-of-sight relative dynamics between the receiver and the SV. If the receiver does not simultaneously adjust (or tune) its replica carrier signal during the code-phase (or range-phase) dimension acquisition and tracking process so that the replica carrier signal matches the frequency of the desired SV carrier, the signal correlation process in the code-phase dimension is severely attenuated by the resulting frequency response roll-off characteristics of the GPS receiver. This has the consequence that the receiver never acquires the SV.
Further, if the SV signal is successfully initially acquired because the SV code and carrier frequency are successfully replicated during an initial search process, but the receiver subsequently loses track of the SV carrier frequency, then the receiver loses code track as well. Therefore, in the carrier Doppler frequency dimension (i.e., in the carrier-phase dimension), the GPS receiver accomplishes carrier matching (also referred to as “wipeoff”) by first searching for the carrier Doppler frequency of the desired SV and then tracking the SV carrier Doppler state. The GPS receiver typically performs this process by adjusting a nominal carrier frequency of its replica carrier frequency generator to compensate for Doppler-induced effects on the SV carrier signal caused by line-of-sight relative dynamics between the receiver and the SV.
Global Positioning Systems use a multiplicity of satellites to concurrently transmit signals to a GPS receiver to permit position location of the receiver by measurement of differences in times-of-arrival between the multiple transmitted signals. In general, the signals transmitted from the different satellites do not significantly interfere with one another, because they use different PRN codes, unique to each satellite, wherein the PRN codes are nearly orthogonal to one another. This low interference condition depends upon the power levels (amplitudes) of the received signals being similar to one another.
Under certain operating conditions, however, one or more satellite signals can be highly attenuated relative to the other satellite signals. Such an operating condition may arise, for example, from the blockage of certain satellite signals, as may occur in urban canyon environments. Under these conditions, the presence of the strong GPS signals produces interference that can reduce the ability to detect and track weaker GPS signals. As described in an excellent text on GPS systems, entitled Understanding GPS Principles and Applications, edited by Elliott D. Kaplan, published by Artech House, Inc. in 1996, and incorporated herein for its teachings on GPS systems and receivers (referred to hereafter as the “Kaplan” text), because the GPS C/A-code length is a compromise at 1,023 chips, the cross-correlation properties (i.e., the cross-correlation between the unique PRN codes) can be poor under certain operational conditions. Table 1 below shows the C/A-code cross-correlation power for zero Doppler differences between any two codes. Table 1 is adopted from Table 4.7 of the incorporated Kaplan text (appearing at page 115), however it has been revised to reflect the results of simulations performed by the inventors.
As is well known in the GPS receiver design art, cross-correlation spurs are generated when an interfering SV transmits at certain frequencies relative to the search frequency of the target SV. As shown in Table 1 below, when the Doppler difference is zero Hz, the cross-correlation power adopts one of three specific values, relative to the power of the stronger SV (assuming that chip boundaries are synchronized for the two SVs). The C/A-code cross-correlation functions have peak levels that can be as poor as −24 dB with respect to its maximum autocorrelation for a zero Doppler difference between any two codes.
TABLE 1C/A-Code Maximum Cross-Correlation Power(Zero Doppler Differences)Cumulative ProbabilityCross-Correlation forof OccurrenceAny Two Codes (dB).12−23.9.25−24.21.00−60.2
Because the C/A codes have a period equal to 1 ms, the most significant cross-correlations occur when the Doppler difference between interfering SV and the target SV is a multiple of 1 kHz. There are also weaker cross-correlations at other frequencies. Table 2 (reproduced from Table 4.8, on page 116, of the incorporated Kaplan text) below shows the cross-correlation distribution for Doppler differences of 1, 2, 3, 4 and 5 kHz. As shown in Table 2, when the Doppler difference is nonzero and a multiple of 1 kHz, the worst-case cross-correlation power is −21.1 dB (relative to the interfering SV).
TABLE 2C/A-Code Maximum Cross-correlation Power for Two SV's with NonzeroDoppler Differences (Increments of 1 kHz Doppler Differences)CumulativeCross-Cross-Cross-Cross-Cross-Probability ofCorrelation @Correlation @Correlation @Correlation @Correlation @OccurrenceΔ = 1 kHz (dB)Δ = 2 kHz (dB)Δ = 3 kHz (dB)Δ = 4 kHz (dB)Δ = 5 kHz (dB)0.001−21.1−21.1−21.6−21.1−21.90.02−24.2−24.2−24.2−24.2−24.20.1−26.4−26.4−26.4−26.4−26.40.4−30.4−30.4−30.4−30.4−30.4
This cross-correlation can cause false acquisitions under certain Doppler difference and antenna gain conditions. For example, as described in the incorporated Kaplan text, an unwanted SV can have a C/A-code signal that is approximately 7 dB stronger than the desired SV C/A-code signal. If the desired SV is low on the horizon, increased multi-path loss is present, as well as reduced GPS receiver antenna gain, which results in a typical net loss of 4 dB. If the unwanted SV is higher in elevation, the atmospheric loss is reduced and there is increased GPS receiver antenna gain. This results in a typical net gain of 3 dB. The difference in SV elevations reduces the signal separation by an additional 7 dB. The antenna SV array gain variations as a function of the differences in user elevation angles to the two SVs also contribute up to an additional 2 dB of gain to the unwanted stronger SV signal. Under signal conditions between the two SVs where there is only 21 dB of C/A-code discrimination during the cross-correlation process, the difference between the desired SV signal and the unwanted signal is only 5 dB (21 dB−16 dB).
As a consequence of these cross-correlations, the GPS receiver may falsely acquire the stronger SV signal. Therefore, a method of discriminating and mitigating these cross-correlation effects in a GPS receiver is desired. The GPS receiver design should implement sophisticated C/A-code search procedures that avoid “sidelobe” and unwanted SV acquisitions. However, the cross-correlation mitigation method and apparatus should be inexpensively and easily implemented in a GPS receiver.